Ex situ EPR investigation of MoO3/Al2O3 catalysts for propylene metathesis

A series of propylene metathesis MoO3/Al2O3 catalysts (2–10 wt. % of MoO3) activated in Ar and under ex situ reaction conditions was studied by EPR spectroscopy. Similar EPR signals from surface molybdate clusters (Mo5+) were observed for each sample. The concentration of pentavalent molybdenum in paramagnetic form determined by EPR increases with an increase in the MoO3 content from 2 to 5 wt. %, then changes slightly. It was also found that the activity of the catalysts in the propylene metathesis reaction is proportional to the increment in the concentration of Mo5+ paramagnetic species determined after contact with propylene. This indicates that higher oxidation states of molybdenum form active paramagnetic complexes during the reaction with propylene.